1. Field of the Invention
The present invention relates to methods of processing signal samples representative of a colour video image to produce a legalised colour version of the image. Furthermore, the present invention relates to apparatuses for processing signal samples representative of a colour video image to produce a legalised colour version of the image.
2. Description of the Prior Art
It is well known that the colours of the rainbow, which correspond to light with a range of wavelengths which is visible to the human eye, can be represented from combinations of the colours red, green and blue. For this reason colour television and video images are generated by separating the red, green and blue components of the images and sampling these components at spatially separated sampling points within the image. For example, colour television cameras are provided with a dichronic element which separates the colours of an image formed within a field of view of the camera into red, green and blue components. Each of the red, green and blue components of the image is sampled in two dimensions in accordance with a row-by-column de-composition of the image. Each row is sampled at regularly displaced sampling points to produce a number of samples representing the row which produces the row-by-column de-composition of the image. These sampling points are known to those skilled in the art as pixels. Each of the samples represents one of the red, green and blue components of one of the pixels which make up the image.
The colour image may be re-generated from the signal samples using a colour visual display unit, by separating the signal samples representing the red, green and blue components of the pixels and feeding each respectively to one of three image generators. Each of the image generators operates to reconstruct, row-by-column, a version of the image for one of the three colours of red, green or blue which are super-imposed on a colour screen. By producing the red, green and blue components of each pixel at positions on the screen corresponding to the positions of the pixels from which the colour image was sampled, the colour image is re-generated. Since each pixel is comprised of red, green and blue components, the relative intensity of these components produces a mixture of red, green and blue light which represents the colour at the corresponding point of the image. The mixture of the red, green and blue components can therefore reproduce any of the colours of the original colour image, which can be any of the colours of the rainbow. A combined effect of the three image generators is therefore to reproduce a version of the colour image which is representative of the colour image formed within the field of view of the television camera.
Representing a colour image as red, green and blue signal samples provides a facility for transmitting, recording and reproducing the colour image in some way. However, in order to reduce an amount of information which must be transmitted in order to convey the colour image, known television transmission techniques and video image recording techniques convert the red, green and blue signals into colour difference signals, which are generally comprised of a luminance and a first and a second chrominance signal. The luminance signal is, for example, formed by combining the red, green and blue signal components of a pixel into a single component representative of the relative strength of the light in the image at the pixel location. The first of the chrominance signals is generated by forming a difference between the luminance signal and the red signal, and the second chrominance signal is formed from the difference between the luminance signal and the blue colour signal.
The colour difference signal format is one example of a signal format which forms a signal space in which the pixels of a colour video image can be represented, but which does not directly correspond with the red, green and blue components from which the colour video image was generated. As a result, not all values of the colour difference signal components representing a pixel within the colour difference space correspond to pixels within the signal space formed from the red, green and blue components of the colour image. For example, if the luminance component is at its minimum value of zero, then any non-zero value of the two chrominance signal components will result in a signal value which does not fall within the red, green and blue colour reference space. Similarly, if the luminance signal is at a maximum value which corresponds to white light, then any non-zero values of the two chrominance signals will also not fall within the red, green and blue reference space.
Any colour which does not fall within the red, green and blue reference space is an illegal colour. For the example of colour difference signals, any combination of the three components of the colour difference signals which results in a value which does not fall within the red, green and blue colour reference space will be an illegal value. Such illegal colour values can be produced when the colour images are transmitted or processed as, for example, colour difference signals. For example, video signals are often processed in this format to introduce video effects such as colour wash effects. As a result, values of the three colour reference space components can be produced which are illegal values within the red, green and blue reference space. If these illegal colour values are displayed within a colour image, colours can result which do not match with the legal parts of the image. The colour visual display unit reproducing the image may hard limit the colour value to a maximum value of the component which can be displayed, and the illegal pixels may be reproduced or processed in an unpredictable way.
In an article entitled xe2x80x9cLimiting of YUV Digital Video Signalsxe2x80x9d by V G Devereux from the Research Department, Engineering Division, of the British Broadcast Corporation dated December 1987, a method of converting illegal colour pixels in a form of YUV colour difference signals into legal colour pixels with respect to the red, green and blue (RGB) colour reference space is disclosed. This method changes the components of the pixels in the YUV colour difference space with respect to each other in order to convert the pixel in the corresponding red, green and blue colour reference space into a legal pixel.
Having regard to the above discussion, it will be appreciated that there is a general requirement to provide a method of processing colour video images in order to convert reliably illegal colour pixels of the images into legal colour pixels.
According to the present invention, there is provided a method of processing input signal samples representative of at least part of a colour video image to produce legalised signal samples representative of a legal colour version of the image, the method comprising the steps of generating an over sampled version of the input signal samples by generating at least one extra signal sample for each base input signal sample, generating adjustment factors from the input signal samples, which when combined with the input signal samples have an effect of converting illegal colour pixels of the colour video image into legal colour pixels, combining the adjustment factors with the input signal samples to produce the legalised colour signal samples, and decimating the over sampled legalised colour signal samples to produce decimated legalised signal samples having a sampling rate corresponding to that of the base input signal samples, by selecting signal samples from the over sampled version of the legalised colour signal samples which have not changed with respect to the corresponding input signal samples, and if the signal samples have changed combining a plurality of the over sampled legalised colour signal samples to produce the decimated legalised signal samples.
It has been discovered that illegal colours can be produced as a result of distortion caused by aliasing errors. The distortion is produced by high frequency components of the video image in the analogue domain which are outside a maximum frequency which can be represented in accordance with a sampling rate of the input signal samples. This produces distortion in the video image as a result of aliasing errors.
To provide an improvement by reducing the effect of this distortion, an over-sampled version of the input signal samples is generated so that these out-of-band components in the video image appear within the in-band components of the over-sampled version of the input signal samples. This provides a further advantage in representing the analogue video signal more accurately because the sampling points of the video image at the lower sampling rate can fall at positions which do not correspond to a maximum of the video signal. The over sampled version therefore provides a better representation of the video signal, because there is an increased likelihood that maxima of the video signal are close to or correspond to the temporal sampling positions. The video image is then legalised in this over sampled form, by generating and applying adjustment factors to produce legalised colour signal samples. However, legal colour pixels of the colour video image can become illegal when an over sampled version of the legalised colour signal samples is decimated since decimating the legalised colour signal samples involves representing a plurality of signal samples to as a single decimated signal sample. By selecting the legalised colour signal samples associated with the original sampling positions of the input signal, this distortion is substantially reduced. The term decimating refers to a process in which an over sampled signal is reduced to a version with signal samples having a sampling rate corresponding to that of the original input signal samples. This may involve combining signal samples to produce a composite decimated signal sample for a plurality of signal samples, or this may involve filtering and then dropping the extra signal samples associated with the over sampled version, and forming the decimated version from the samples at the same positions as that of the original signal samples. Decimating in this sense can cause legal colour pixels to become illegal, as this can involve changing some signal samples.
Advantageously, the step of decimating the over-sampled version of the legalised colour signal samples may comprise the steps of determining whether each of the legalised colour signal samples and the extra-legalised colour signal samples associated with each of the legalised colour signal samples were changed with respect to the corresponding input signal samples and the extra input signal samples when combined with the adjustment factors, and if at least one of each input signal sample and the associated signal samples have changed with respect to the legalised colour signal samples performing the steps of combining the legalised colour signal sample and the extra colour signal samples to form a corresponding decimated legalised colour signal sample, or if neither the legalised colour signal sample nor the associated extra-legalised colour signal samples have changed, discarding the associated extra-legalised colour signal samples and forming the decimated signal sample from the legalised colour signal sample. Decimating the legalise colour signal by selecting the base legalised colour signal sample, that is the signal corresponding to the sampling point of the input signal samples, a possibility of an illegal pixel being generated by combining the base and the associated extra legalised colour signal sample is substantially reduced.
As explained above, although the input signal samples which are representative of the colour video image may have values with respect to a signal space which is different from the red, green and blue signal space an example embodiment of the invention finds particular application where the input signal samples are colour difference signal samples having luminance and two colour difference signal components. As such, in the case where the adjustment factors are calculated and applied with reference to the red, green and blue colour reference space, the step of combining the adjustment factors with the input signal samples comprises the steps of converting the input colour difference signal samples into a colour reference signal samples having values with respect to three orthogonal colour reference axes of red, green and blue light, combining the colour reference signal samples with the adjustment factors and converting the combined colour reference signal samples into colour difference signal samples.
Although the adjustment factors may be digital values which are added to the input signal samples in order to generate the legalised colour signal samples, in a preferred embodiment, the adjustment factors are scaling factors and the step of combining the adjustment factors with the input signal samples comprises the step of multiplying the adjustment factors with the input signal samples.
Accordingly to an aspect of the present invention, there is provided an image processing apparatus according to patent claim 6. Further features and aspects of the image processing apparatus are provided in the appended claims.